The present invention relates to a method for processing a stream of LNG obtained by means of cooling using a first refrigeration cycle, the method being of the type comprising the following steps:
(a) the stream of LNG which has been brought to a temperature of less than −100° C. is introduced into a first heat-exchanger;
(b) the stream of LNG is sub-cooled in the first heat-exchanger by means of heat-exchange with a refrigerating fluid in order to form a stream of sub-cooled LNG; and
(c) the refrigerating fluid is subjected to a second semi-open refrigeration cycle which is independent of the first cycle.
U.S. Pat. No. 6,308,531 discloses a method of the above-mentioned type, in which a stream of natural gas is liquefied using a first refrigeration cycle which uses the condensation and evaporation of a mixture of hydrocarbons. The temperature of the gas obtained is approximately −100° C. Then, the LNG produced is sub-cooled to approximately −170° C. using a second refrigeration cycle of the type referred to as a semi-open “inverted Brayton cycle” comprising a stage compressor and a gas expansion turbine.
A method of this type is not entirely satisfactory. The maximum yield of the inverted Brayton cycle is limited to approximately 40%. Furthermore, the operation thereof in a semi-open cycle is difficult to implement.